Add core runtime: event bus, services, app lifecycle

lib/core is hardware-independent and unit-tested on the host:
- EventBus: fixed-size thread-safe queue, delivered on the UI task by
  dispatch(); drops newest when full and counts drops
- Service / ServiceManager: cooperative ticking at per-service intervals,
  no catch-up bursts, safe across millis() wraparound
- App / AppManager: one foreground App, Home always to Launcher, Back
  offered to the App first, hidden Apps, redraw requests
- KeyEvent: logical keys for the upcoming input layer

Firmware main loop now ticks services and dispatches events.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EhqxQ49eCju4CzKYNjZzwT

lib/core/src/app.h

@@ -0,0 +1,37 @@
+#pragma once
+
+#include "key_event.h"
+
+namespace roro {
+
+class Canvas;  // provided by the widget kit
+
+// A foreground, user-facing program (see CONTEXT.md). Exactly one App is on screen at a time.
+class App {
+   public:
+    virtual ~App() = default;
+
+    virtual void onEnter() {}
+    virtual void onExit() {}
+
+    // Return true if the key was consumed. An unconsumed Back leaves the App.
+    virtual bool onKey(const KeyEvent& event) {
+        (void)event;
+        return false;
+    }
+
+    virtual void draw(Canvas& canvas) = 0;
+
+    void requestRedraw() { redraw_ = true; }
+
+    bool consumeRedraw() {
+        bool r = redraw_;
+        redraw_ = false;
+        return r;
+    }
+
+   private:
+    bool redraw_ = false;
+};
+
+}  // namespace roro